ApplicationLogs Plugin: do not replace invalid ApplicationLog's stackitem with Null

[AnnaShaleva]

Summary or problem description
The original issue is described in #3299 (comment). If ApplicationLog has stackitem that fails to be serialized, it's replaced by the Null stackitem:

neo/src/Plugins/ApplicationLogs/Store/LogStorageStore.cs

Line 164 in 6e8c730

_snapshot.Put(key, BinarySerializer.Serialize(StackItem.Null, ExecutionEngineLimits.Default with { MaxItemSize = (uint)Settings.Default.MaxStackSize }));

It's not correct from the end-user POW since there's no way to distinguish the "good" original stackitem from the result of invalid seialization.

Do you have any solution you want to propose?
Use a special stackitem stub with InvalidT type which can't be produced by any means other than exception during serialisation. With this special type there's no doubts from the user side whether there was an exception during serialization or not. This approach is implemented in NeoGo, see https://github.com/nspcc-dev/neo-go/blob/cf4d4a2611209d8d085ccec6e1f1b8956006a577/pkg/vm/stackitem/serialization.go#L144.

Where in the software does this update applies to?

• Plugins

[dusmart]

I appreciate every effort that makes exception related message clear. Thanks.

[cschuchardt88]

I think null stackitem is appropriate choice for something that error. Seeing how the state can't be saved or there is no state.